It is known to use residual-moisture photographs (wet-film technique) for making visible and recording steady-state boundary layer flow in gases and liquids, using photographic film (German Pat. No. 2,133,834).
This photographic method is based on the evaporation of a liquid from a photographic gelatin film in an air stream. The rate of evaporation is determined by the local layer thickness of Prandtl's boundary layer.
A commercial photographic film is swelled with water and exposed in the flow channel. The moisture distribution remaining in the photographic gelatin layer after it is surface dried by conducting an inert gas over it, i.e., the residual moisture profile, is transformed, by conducting a photochemically reactive gas such as hydrogen sulfide gas over it, into a latent image which can be developed photographically and is converted by a photographic developer into a moisture-analog half-tone photograph.
Besides the foregoing "sulfide seeding method", in which the drying profile is made visible by hydrogen sulfide gas, the drying profile can also be made visible by post-exposure, generating a moisture-analog silver half-tone profile upon post-exposure from the photographic developer absorbed in the residual-moisture profile.
The practical application of this method, which usually works with concentrated H.sub.2 S gas, is often difficult. Dark room conditions and a closed hood are required. As the contrast range of the photographs is relatively small, an evaluation cannot be made directly from the original by means of equidensity images but only via an intermediate copy.